The field of the invention relates to optical fibers used for the transmission of light pulses, and in particular to preforms or fibers having multiple optical cores.
Glass fibers have conventionally been formed by softening and then drawing high-silica-content glass rods known as "preforms". The center of the preform is made of a glass composition desired for the core of the fiber. It is usually nearly pure silica with a few percent of a dopant. The outer portion of the preform is composed of glass suitable for the fiber cladding. An appropriate dopant, such as boron, Trichloride is injected into silica tetrachloride to yield the cladding material. The modified chemical deposition process has been found to be most advantageous in the fabrication of high-silica preforms. It is comparatively simple and allows the injection of relatively high percentages of dopants. The resultant product is known as a CVD preform.
Use of two slightly different types of glass for the core and cladding materials give the core a slightly higher index of refraction than the cladding. The different indices allow light to be reflected along the entire length of an optical glass fiber.
It is often desirable to be able to simultaneously send signals in both directions through a cable. This has traditionally required the use of bidirectional couplers in conjunction with the optical cable. It is also necessary in many circumstances to determine whether a cable has been tampered with.
A tamper-proof cable has been proposed which comprises concentric layers of core and cladding material, respectively. A signal is transmitted through the central core which is encircled by a first layer of cladding material. A monitoring signal is transmitted through a layer of core material disposed exteriorly and concentric to the first layer of cladding. This second core is also sheathed by cladding material.
If an attempt is made to tamper with the cable, the monitoring signal will be interrupted or attenuated. This allows the detection of such tampering.